The present invention relates to a variable valve system for an internal combustion engine.
More specifically, the present invention relates to a variable valve system which is provided with a plurality of variable gears for controlling valve lift characteristic and the like of an engine valve such as an intake valve and an exhaust valve.
U.S. Pat. No. 6,123,053 (equivalent of Japanese Patent Unexamined Publication No. 2000-38910 which is applied by the applicant of the present invention) discloses a variable valve system (referred to as xe2x80x9cVARIABLE VALVE ACTUATION APPARATUSxe2x80x9d). The variable valve system according to the above related art is applied to a movable valve gear which is provided with two intake valves for one cylinder. The variable valve system has a first variable gear and a second variable gear, each for variably controlling a valve lift characteristic of one of the respective two intake valves, namely, a first intake valve and a second intake valve, in such a manner that a lift of the first intake valve becomes different from a lift of the second intake valve, to thereby achieve engine performance in accordance with engine operating condition.
According to the above related art, however, only one control shaft is used for rotatably controlling the lift of each of the first variable gear and the second variable gear. Thereby, the two variable gears interlock with each other. In other words, the valve lift characteristic of one engine valve becomes a determinant of the valve lift characteristic of the other engine valve, causing insufficiency in engine performance in accordance with engine operating condition.
More specific description referring to FIG. 7 of the above related art is as follows. When the control shaft is rotated in a first direction so as to increase the lift, each of the first intake valve and the second intake valve has a large lift (same as each other). When the control shaft is rotated in a second direction opposite to the first direction, each of the first intake valve and the second intake valve has a small lift becoming smaller by degrees. With this, a lift difference is caused between the first intake valve and the second intake valve. The thus caused lift difference is gently increased.
Herein, engine perforce at low engine speed and light load is described as follows: The above increased lift difference between the first intake valve and the second intake valve encourages an intake air flow, to thereby improve combustion. Thereby, fuel consumption can be reduced in engine operating area.
On the other hand, engine performance at low engine speed and heavy load is described as follows: The gas flow causes an intake air loss (equivalent to the gas flow). Therefore, the lift must be increased so as to reduce the lift difference. However, after the piston passes over the bottom dead center, the increased lift difference ousts the mixture (that has been once introduced into the cylinder) at the latter period of lifting operation. Thereby, intake air filling efficiency is reduced, and output torque is likely to decrease. In high lift area, the lift difference cannot be reduced. Therefore, it is difficult to improve intake air flow effect at high engine speed area requiring high lift.
It is an object of the present invention to provide a variable valve system for an internal combustion engine.
According to the present invention, there is provided a variable valve system for an internal combustion engine. The variable valve system comprises a plurality of valves provided for one cylinder of the internal combustion engine. The plurality of the valves are disposed on one of an intake side and an exhaust side of the one cylinder. The plurality of the valves comprises a first valve, and a second valve. The variable valve system further comprises a first variable gear for variably controlling at least a lift of a valve lift characteristic of the first valve, and a second variable gear for variably controlling at least a lift of a valve lift characteristic of the second valve. The first variable gear and the second variable gear operate independently of each other.
The other objects and features of the present invention will become understood from the following description with reference to the accompanying drawings.